1. Field of Invention
This invention relates to data acquisition for digital systems and more specifically to analog to digital data conversion.
2. Description of Prior Art
Of all interfaces between different types of systems, the most important and most difficult is usually a link between the analog and digital sections of the system. The interface between digital circuit and analog circuits include a digital-analog converter or D/A converter. Likewise the interface between the analog circuits and the digital circuits includes an A/D converter. An ideal A/D converter changes an analog signal into a digital signal without delay and without error. Of course, such ideal converters are not available. However A/D converters with some delay and limited accuracy are available and are priced according to their accuracy and speed. The techniques used for A/D and D/A conversions are many. One paper that includes a thorough description of many A/D and D/A conversion techniques is an article by Bernard Gordon in the IEEE Transaction of Circuit Systems, CAS-25, July 1978, entitled "Linear Electronic Analog/Digital Conversion Architectures, Their Origins, Parameters, Limitations, and Applications." An additional paper in the same publication also describes MOS analog to digital devices. This paper is entitled "ALL-MOS Analog-Digital Conversion Techniques" by Paul R. Gray and David G. Hodges in the IEEE Transactions on Circuits and Systems, Volume CAS-25, 7 July 1978. This paper specifically describes the technique that is used in this invention under the heading of integrated circuit successive approximation converters. This technique includes charge redistribution which is described in Paul R. Gray's previous paper entitled "All-MOS charge Redistribution Analog-to-Digital Conversion Techniques Part I" by James L. McCreary and Paul R. Gray, published in the IEEE Journal of Solid State Circuits, Volume SC-10, Dec. 6, 1975 in which charge redistribution is discussed using a bank of weighted capacitors.
Problems arise with the use of weighted capacitors because the fabrication accuracy does not assure the accuracy needed for application. In the past, capacitors have been trimmed in an attempt to adjust them to obtain satisfactory accuracy. One technique that includes resistors and capacitors to obtain a higher resolution disclosed in the paper, "An INMOS 12.sub.b Monotonic 25 microsecond A/D converter" by Bahram Fotouhi and David A. Hodges presented at the 1979 IEEE International Solid States Circuits Conference, page 186. The technique disclosed shows the use of a resistor bank to effectively adjust the charge on capacitors. Another error correcting technique is disclosed in a paper entitled "An Error Correcting 14 Bit-20 Microsecond CMOS A/D Converter" by Ziya G. Boyacigiller, Basil Weir and Peter D. Bradshaw. Presented at the 1981 IEEE International Solid State Circuits Conference, this technique includes an error correcting circuitry that uses a redundant D/A with a successive approximation register.